3988
J . Am. Chem. SOC.1992, 114, 3988-3989
appropriate geometry to form an intramolecular hydrogen bond, there is insufficient data on the gas-phase basicities of guanidine compounds to determine whether cyclization occurs for this amino acid on the basis of the measurements reported here. Table I1 compares prior measurements of the proton affinities of six amino acids with the measurements reported here. To facilitate the comparison, the proton affinity values in the table are referenced to the proton affinity ladder used in the earliest study.2 We find excellent agreement with Meot-Ner's measurements of the proton affinities of alanine, valine, leucine, and proline and close agreement for glycine and phenylalanine.2 Likewise, we find excellent agreement with the proton affinity value of alanine reported by Locke and M ~ I v e r .Our ~ value for the proton affinity of glycine is lower than, but similar to, that measured by Locke and McIver. The comparisons show no systematic differences between ours and prior measurements. Our relative ordering of the proton affinities of the amino acids agrees with that proposed by Bojensen with a few notable exceptions.I3 Bojensen places histidine as the second most basic amino acid, while we find it sixth on our list. In Bojensen's study, glutamine is more basic than glutamic acid, opposite to our finding. We find alanine, methionine, and threonine to be more basic than does Bojensen. While the exact reason for the differences between the two measurements is not known, it should be emphasized that the measurements reported here are based on well-established ionmolecule techniques and theory. Bojensen relates ion abundances in a metastable ion mass spectrum to thermodynamic properties of the product ions, a technique proposed by Cooks and -workers for measuring relative proton affinities of closely related, monofunctional compound^.'^ The discrepancies between our measurements and those of Bojensen may be a limitation of Cook's method, which has never been tested with structurally diverse, polyfunctional molecules such as the amino acids.
Acknowledgment. This work was supported by the National Science Foundation (CHE-9024922). Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. (13) Bojensen, G. J. Am. Chem. SOC.1987, 109, 5557-5558. (14) McLuckey, S. A,; Cameron, D.; Cooks, R. G. J . Am. Chem. SOC. 1981, 103, 1313-1317.
Different Reaction Paths Taken by Hydrogen Isotopes Zhennan Wu, John M. Stadlbauer,t and David C. Walker* Chemistry Department and TRIUMF University of British Columbia Vancouver, V6T 1 Y6 Canada Received September 6, 1991
Kinetic isotope effects usually reveal differences in degree but not in kind. However, in studying the chemistry of muonium (hydrogen's light isotope with a positive muon as nucleus) we find it to behave as a nucleophile in its reactions with aromatic Nheterocyclic solutes in water. This contrasts with the electrophilic character of IH.'s2 Muonium (Mu) has oneninth the atomic mass of 'H but virtually the same reduced mass, ionization energy, size, etc. as all hydrogen isotopes. Hitherto Mu has shown kinetic and spectroscopic isotopic effects which are understandable in terms of its different atomic mass from that of 'H;3but in the present 'Permanent address: Chemistry Department, Hood College, Frederick, MD 21701. ( 1 ) (a) Neta, P.; Schuler, R. H. J Am. Chem. SOC.1972, 94, 1056. (b) Neta, P. Chem. Rev. 1972, 72, 533. (2) Pryor, W. A.; Lin, T. H.; Stanley, J. P.; Henderson, R. W. J . Am. Chem. SOC.1973, 95, 6993. (3) (a) Walker, D. C. Muon and Muonium Chemistry; Cambridge University Press: Cambridge, 1983. (b) Walker, D. C. Can. J . Chem. 1990, 86, 1719.
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Table I. Rate Constants" for the Reaction of Muonium Atoms with Aromatic N-Heterocyclic Solutes in Water at -295 K. Comparison with Published Data on 'Hb solute kM kM/C kH kH/N km/kH benzene 33 f 3 5.5 (1.5)c 3.7 9 pyridine 58 f 4 12 7.8d 7.8 7.4 50 f 3 12.5 pyridazine 2.7' 1.4 18.5 37 f 2 pyrimidine 9 0.92' 0.46 40.2 77 f 5 pyrazine 19 3 0 1.5 25.7 "All k values are in units of 10' dm3 mol-' s-I; kM/C and kH/N are described in the text. See footnotes c to g for pH dependence. bReference 8. CCalculatedas kH per c atom. dThe 7.8 refers to natural pH. At pH -1, however, kH = 1.7 X lo* M-' s-' for pyridine where >99% of the solutes were protonated; so protonation decreases kH by -4.5-fold. This contrasts with Mu,where kMincreased from 58 to 74 (XlO*) on changing from pH -7 to pH 1.2, for pyridine. Thus kM/kH increases from 7.4 at p H -7 to 44 at pH -1. 'kH measured at pH -1, where -99% of the solutes have one of their N atoms protonated; so the number of unprotonated N atoms is half that relevant to the kM data. 'kH measured at p H -1, where -65% of the solutes have one of their N atoms protonated. gkH measured at p H 1, where -30% of the solutes have one of their N atoms protonated.
-
study it shows a different type of reaction. Recently we found that Mu produces the free radical which arises from addition into the aromatic ring of pyrazine at its C atoms! Now we report the rate constants measured for that reaction and Mu's reaction with other N-heterocyclic compounds. Table I gives our observed rate constants (kM) for reaction of Mu with benzene, pyridine, and the 1,2-, 1,3-, and 1,4-diazines. These data were obtained by measuring, the chemical decay rate of muonium using the muon-spin-rotation technique5 on millimolar aqueous solutions of these compounds at natural pH. They are compared in Table I with the published data for reaction of 'H atoms with the same solutes under similar conditions. Since Mu adds to a ring C : whereas 'H attaches at the more electronegative N atoms,6 these data have been normalized in columns 3 and 5 of Table I by dividing kMby the number of C sites on the solutes and kH by the number of N sites. Mu is seen to react 3.7 times faster than 'H with benzene. This is not inconsistent with Mu's 3-fold-higher mean thermal velocity stemming from its one-ninth atomic mass: a kinetic isotope effect The overall rate constants expected for diffusion-limited reacti~ns.~ ratio, k M / k Hthen , increases to 7.4 due to the presence of one ring N, and up to 40 with two N atoms. This ratio rises partly by kM increasing and partly by kH decreasing. The enhancement of kMby ring nitrogens implies that Mu is "nucleophilic", because N draws electron density from the C atoms where Mu reacts. Preliminary results even suggest that there is a direct proportionality between -log ( k M )and the Hiickel molecular orbital localization energy on each C.' By contrast, the presence of the ring N of pyridine seems to switch H's reaction site from C to N (as seen through ESR data6). This results in a minor overall decrease in kH (but an increase per atom where reaction takes place). The presence of a second N atom then reduces the rate further. Also, 'H has already been shown to display a negative Hammett p parameter,',* and the differences between the diazines have been interpreted as consistent with this electrophilic character.' There is additional information to be gleaned from the pH dependence of these rate constants arising from protonation of the solute. For pyridine, which is the only N-heterocycle for which jumps from 7.4 at pH -7 to 44 at pH data are available, k M / k H 1. Almost all of this jump (see footnotes to Table I) emerges from a decrease in kH. This consequently corroborates the view that reaction of H, but not Mu, occurs on the N atom.
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(4) Wu, Z.; Barnabas, M. V.; Stadlbauer, J. M.; Venkateswaran, K.; Porter, G. B.; Walker, D. C. J . Am. Chem. SOC.1991, 113, 9096. (5) Cox, S. F. J. J. Phys. C 1987, 20. 3187. (6) Barton, B. L.; Fraenkel, G. F. J . Chem. Phys. 1964, 41, 1455. (7) Z. Wu et al., preliminary unpublished data. (8) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.;Ross,A. B. Nafl. Stand. Ref. Data Ser. (US., Natl. Bur. Stand.) 1988, NSRDS-NBS.
0 1992 American Chemical Society
J. Am. Chem. SOC.1992, 114, 3989-3990
We are left with the overall conclusion that Mu and H, despite having binding energies and polarizabilities within 0.5% of each other, respond in opposite senses toward the uneven electron distributions of N-heterocyclic aromatic rings. This results in different reaction mechanisms predominating and therefore different reaction products being formed.
Acknowledgment. We appreciate collaborations with Drs. M. V. Barnabas, G. B. Porter, B. W. Ng, and K. Venkateswaran. J.M.S. thanks the Hood College Board of Associates for financial assistance. Registry No. H, 12385-13-6; muonium, 12587-65-4; benzene, 71-43-2; pyridine, 110-86-1; pyridazine, 289-80-5; pyrimidine, 289-95-2; pyridine conjugate acid, 16969-45-2; pyridazine conjugate acid, 17009-97-1; pyrimidine conjugate acid, 17009-95-9; pyrazine conjugate acid, 1700993-7; pyrazine, 290-37-9.
3989
Scheme I Li (dispersion)
(EtZN),Phs .,,Si CI
*
( EtZN),Pb.,,SiLi
THF, 0 “c, 4 h
4(n=l) 5 ( n = 2)
l ( n E 1 ) 98% 2 ( n = 2 ) 97% Li (dispersion)
(Et2N)MePhSiCI
c
(Et2N)MePhSiLi
THF, 0 “c, 24 h
6
3
80%
Scheme I1 (Et2N)PhzSiLi + CISiMe2R
c
1
0 “c.0.5- 2 h Ad)l
(Et2N)Ph2SiSiMe2R
7 R=H 8 R=NE4 9 R=Oi.Pr 1 0 R=CH=CH2
The First Stable Functional Silyl Anions:
OC2h
-
CiPh2SiSiMepX
11 X = H 84% 1 2 x=u 06%
85% 91% 7%
60%
(Aminosily1)lithiums Kohei Tamao,* Atsushi Kawachi, and Yoshihiko Ito* Department of Synthetic Chemistry Faculty of Engineering Kyoto University, Kyoto 606, Japan Received January 2, I992 Carbanion chemistry plays a central role in the whole field of organic chemistry:’ many of a large number of carbanions are functional carbanions stabilized by heteroatoms,2 including cyheteroatom-substituted carbanions (A). Reported herein are our initial results on the analogous species in silicon chemistry (B).
1,) 12aawn-4 ~
l ( x 2 ) + C12SiMez
O”c,18h
2)W G i ,NH&I r.f., 24 h
-
Me2
(CPrO)Ph2Si-Si-SiPh2(Oi.Pr)
04),7h
13 overall56%
14 4 7 %
Scheme 111
\ /x
(EtZN)Ph@
1
(E12N)ZPhSiU
(EhN)MePhSiU
2
3
Silyl anions have been studied much less extensively than carbanion^.^,^ Synthetically useful silyl anions have long been (1) (a) Comprehenriue Carbanion Chemistry; Buncel, E., Durst, T., Eds.; Studies in Organic Chemistry 5; Elsevier: Amsterdam, 1980, 1984; Parts A and B. (b) Wakefield, B. J. Organolithium Methods; Academic Press: London, 1988. (c) Williard, P. G. In Comprehenriue Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 1, pp 1-47. (2) Reviews: (a) Gschwend, H. W.; Rodriguez, H. R. Org. React. 1979, 26, 1. (b) Beak, P.; Zajdel, W. J.; Reitz, D. B. Chem. Rev. 1984, 84, 471. (c) Bcche, G. Angew. Chem., Int. Ed. Engl. 1989, 28, 277. a-Alkoxy carbanions: (d) Sawyer, J. s.;Kucerovy, A,; Macdonald, T. L.; McGarvey, G. J. J . Am. Chem. SOC.1988,110, 842 and references cited therein. a-Amino carbanions: (e) Pearson, W. H.; Lindbeck, A. C. J . Am. Chem. SOC.1991, 113, 8546 and referenced cited therein. (3) Reviews: (a) Wittenberg, D.; Gilman, H. Q. Reu. 1969, 13, 116. (b) Davis, D. D.; Gray, C. E. Organomet. Chem. Reo. A 1970, 6, 283. (c) Fleming, I. In Comprehenriue Organic Chemistry; Barton, E., Ollis, W. D., Eds.; Pergamon Press: Oxford, 1979; Vol. 3, pp 664-669. (d) Fujita, M.; Hiyama, T. J . Synth. Org. Chem., Jpn. 1984, 42, 293. (e) Lambert, J. 8.; Schulz, W. J., Jr. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, England, 1989; pp 1007-1010. (4) Dilithio- and tetralithiosilanes: (a) Mehrotra, S. K.; Kawa, H.; Baran, J. R., Jr.; Ludvig, M. M.; Lagow, R. J. J . Am. Chem. SOC.1990,112,9003 and references cited therein. Theoretical studies on silyl anions: (b) Hopkinson, A. C.; Lien, M. H. Tetrahedron 1981,37, 1105. (c) Magnusson, E. Tetrahedron 1985,41,2945. (d) Damewood, J. R., Jr.; Hadad, C. M. J. Phys. Chem. 1988,92, 33. (e) Hopkinson, A. C.; Rodriquez, C. F. Can. J . Chem. 1990, 68, 1309. (f) Schleyer, P. v. R.; Clark, T. J. Chem. SOC.,Chem. Commun. 1986,1371. (g) Schleyer, P. v. R.; Reed, A. E. J . Am. Chem. SOC. 1988, 110, 4453. (h) Apeloig, Y. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, England, 1989; pp 201-204. NMR studies on phenylsilyl anions: (i) Olah, G. A.; Hunadi, R. J . Am. Chem. Soc. 1980, 102, 6989. (j)Buncel, E.; Venkatachalan, T. K.; Eliasson, B.; Edlund, U. J . Am. Chem. SOC.1985, 107, 303. (k) Edlund, U.; Lejon, T.; Venkatachalam, T. K.; Buncel, E. J . Am. Chem. Soc. 1985, 107, 6408.
THF/MeOH, overall 83%
room temp, 12h
67%
limited to only several simple triorganosilyl anions such as Ph,Si-,S“ Ph2MeSi-,6 PhMezSi-,6 Me3Si-,’ and (Me3Si)3Si-.8 Although three functional silyl anions, C13Si-,9 (RO),Me3-,,Si-,10 and HPh2Si-,” have been reported, the first two are postulated active species generated in situ in the presence of quenching agents, and the last is obtained in about 10% yields and tends to polymerize readily. Thus, stable functional silyl anions have never been prepared. Amino groups (X = NRz) were chosen as functional groups on silicon because of their high stability toward organometallic reagents;12 it was thus anticipated that aminosilyl anions must be stable with respect to both intermolecular substitution and intramolecular cy-elimination to silylene species. It is noted here (5) Benkeser, R. A.; Severson, R. G. J . Am. Chem. SOC.1951, 73, 1424. (6) Gilman, H.; Lichtenwalter, G. D. J. Am. Chem. SOC.1958, 80, 608. (7) (a) Sakurai, H.; Okada, A.; Kira, M.; Yonezawa, K. Tetrahedron Lett. 1971, 1511. (b) Still, W. C. J. Org. Chem. 1976, 41, 3063. (c) Corriu, R. J. P.; Guerin, C.; Kolani, B. Bull. SOC.Chim. Fr. 1985, 974. (8) Dickhaut, J.; Giese, B. Org. Synth. 1991, 70, 164 and references cited therein. (9) (a) Nozakura, S.; Konotsune, S . Bull. Chem. Soc. Jpn. 1956,29,322. (b) Benkeser, R. A. Acc. Chem. Res. 1971,4,94. (c) Li, G. S.; Ehler, D. F.; Benkeser, R. A. Organic Syntheses; Wiley: New York, 1988; Collect. Vol. VI, p 747. (d) Naumann, K.; Zon, G.; Mislow, K. J . Am. Chem. SOC.1969, 91, 7012. (10) Watanabe, H.; Higuchi, K.; Goto, T.; Muraoka, T.; hose, J.; Kageyama, M.; Iizuka, Y.; Nozaki, M.; Nagai, Y. J . Organomet. Chem. 1981,218, 27 and references cited therein. (11) Gilman, H.; Steudel, W. Chem. Ind. 1959, 1094. (12) For example: (a) Colvin, E. Silicon in Organic Synthesis; Butterworths: London, 1981; pp 314-324. (c) Colvin, E. W. Silicon Reagents in Organic Synthesis; Academic Press: London, 1988; pp 87-89.
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